Selective formation of porous silicon

ABSTRACT

A pattern of porous silicon is produced in the surface of a silicon substrate by forming a pattern of crystal defects in said surface, preferably by applying an ion milling beam through openings in a photoresist layer to the surface, and then exposing said surface to a stain etchant, such as HF:HNO 3  :H 2  O. The defected crystal will preferentially etch to form a pattern of porous silicon. When the amorphous content of the porous silicon exceeds 70% the porous silicon pattern emits visible light at room temperature.

ORIGIN OF THE INVENTION

The invention described herein was made in the performance of work undera NASA contract, and is subject to the provisions of Public Law 96-517(35 USC 202) in which the Contractor has elected not to retain title.

TECHNICAL FIELD

The present invention relates to the formation of porous silicon and,more particularly, this invention relates to the selective formation ofpatterns of light-emitting, porous silicon on silicon substrates.

BACKGROUND OF THE INVENTION

Porous silicon produced by etching has been studied for over 30 years.Porous silicon can be oxidized to form silicon-on-insulator (SOI)structures. SOI structures provide electric insulation of active devicesand are radiation hard. Photoluminesce from porous silicon was observedin 1984 (1). Recently, visible luminesces at room temperature fromporous silicon has been observed (2). Stains produced in certainchemical etched (3) have been shown to consist of porous silicon. It hasbeen shown recently that the stain films produce visible light at roomtemperature when radiated with ultraviolet light (5).

Interest in porous silicon has been stimulated by the observation ofvisible luminescence at room temperature which is consistent with theearlier measurements of photoluminescence at 4.2K from the poroussilicon films produced by anodization. Stain etching of silicon incommon aqueous etchants are also known to produce a film of poroussilicon.

Anodic etching was carried out in aqueous HF followed by extendedimmersion in aqueous HF (2). The stain films produced by etching siliconin HF and NaNO₂ or Cr₂ O₃ are similar to the films produced by anodicetching. This is expected since the etching of silicon in HF:HNO₃ :H₂ Osolutions is recognized to follow the same oxidation-reduction reactionchemistry as anodic oxidation. Points on the surface of the siliconbehave randomly as localized anodes and cathodes.

The light-emitting porous silicon film on silicon can be the basis ofoptoelectronic devices such as light-emitting diodes. However, neitherstain etching, nor anodic etching lend themselves readily to standardsolid state device processing. Stain etches and anodic etching arenon-selective, they both etch all surfaces exposed to etchant. It isdifficult to prepare a high resolution pattern such as a diode byetching through an exposed mask.

In order to fabricate devices, the patterns of porous silicon need to beformed on the surface of a silicon substrate and preferably by standardphotoresist and lithographic techniques and materials now used to formdevices on silicon.

The use of anodic etching in aqueous HF to form porous silicon is notsatisfactory. Again the formation of patterns is restricted by theapplication of current to the complete substrate and the difficulty ofselectively forming a high resolution pattern on the surface. Anodicetching is also limited to operation on certain types of doped siliconand to a narrow range of conditions. The method is cumbersome, difficultto control, particularly for n-type structures, and is not compatiblewith many standard optoelectronic silicon device structures.

CITATIONS OF REFERENCES

1. C. Pickering, et al., J. Phys. C.17, 6535 (1984).

2. L. T. Canham, Appl. Phys. Lett. 57, 1046 (1990).

3. R. J. Archer, J. Phys. Chem. Solids 14, 104 (1960).

4. M. T. V. Beale, et al., J. Cryst. Growth 75, 408 (1986).

5. Vasquez, et al., Appl. Phys. Lett. 60, (8) 1992.

STATEMENT OF THE INVENTION

A method of selectively forming a pattern of porous silicon on a siliconsurface is provided in accordance with the invention. The method canoperate on doped or undoped silicon of high or low resistivity and canform the pattern in high resolution on silicon surfaces having differentcrystal orientations. The method can be performed on planar ornon-planar surfaces and does not require use of current toelectrolytically add or remove material.

In order to fabricate devices, a pattern of porous silicon isselectively formed on the surface of a silicon substrate, preferably bystandard resist and lithographic techniques now used to form devices onsilicon.

The process for selective formation of a pattern of light emittingporous silicon is based on the recognition that etching in singlecrystal substances preferentially attacks crystal defects. The inventionexploits this phenomena by 20 forming a pattern of damaged crystal inthe surface of a single crystal. When the surface is exposed to etchant,the damaged pattern forms a film of porous silicon very quickly beforeforming a visible film on the non-damaged areas of the surface adjacentto and surrounding the pattern of porous silicon formed in the damagedcrystal. Patterns of porous silicon are directly produced in highresolution. The method utilizes standard solid state device fabricationtechniques to define the pattern by masking and photolithographic steps.The surface is then contacted with chemical or anodic etchant capable offorming porous silicon. The process of the invention appears to beinsensitive to silicon wafer orientation, dopant type and resistivity ofthe substrate material.

While the pattern could be formed by mechanically scribing the surface,techniques in which the damage is applied to the surface by energeticbeams is more adapted to automated processing and fabrication ofmultiple devices on a single substrate. Standard processing tools thatcould be used to form the pattern of damage in the surface of siliconinclude plasma etching, ion implantation and ion milling.

A wafer is coated with a layer of photoresist. A pattern is formed inthe photoresist layer and windows are opened in the photoresist in theform of a pattern. A beam such as an argon ion milling beam is thenapplied to the surface to damage the crystal exposed through thewindow-pattern. The photoresist is removed and the surface is exposed toetchant. A film of porous silicon is selectively formed in the damagedpattern.

The etchant can be a simple etchant such as a combination ofhydrofluoric acid with an oxidizing acid or salt such as a nitrate,nitrite or chromate or can be hydrofluoric acid electrolyte with thesubstrate connected in an electrolyte cell as an anode.

The anodic etching procedure is more cumbersome, difficult to control,particularly for n-type substrates and is not compatible with manystandard silicon device structures. Chemical stain etches are found toreact well with n-type silicon and p-type silicon and are compatiblewith a wider range of device structures. The method of the inventionprovides the selective production of luminescent material using onlystandard silicon processing apparatus and chemicals which are readilyavailable.

The pattern of porous silicon can be oxidized to form asilicon-on-insulator (SOI) structure. The pattern of light-emitting,porous silicon can directly be fabricated into light-emitting devices.The process of the invention employs standard chemical techniques andchemicals used in silicon devices and can immediately be applied tomanufacture of silicon light-emitting devices used in optical fibercommunications, optical switches or other devices. The invention canalso be utilized to form light emitting diodes which when integratedwith other silicon devices will find use in displays, communication andcomputer applications.

These and many other features and attendant advantages of the inventionwill become apparent as the invention becomes better understood byreference to the following detailed description when considered inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a schematic view of a silicon chip being exposed to a beam;

FIG. 1b is a schematic view of the silicon chip immersed in developerfor the resist layer;

FIG. 1c is a schematic view of the chip being exposed to an energeticbeam through the opening in the resist layer;

FIG. 1d is a schematic view of the silicon chip immersed in etchant;

FIG. 1e is a schematic view of the silicon chip containing a poroussilicon pattern; and

FIG. 2 are photoluminescence spectra of porous silicon films.

DETAILED DESCRIPTION OF THE INVENTION

The invention is based on forming a latent pattern in the surface of asilicon substrate that on exposure to etchant forms a pattern of poroussilicon. The latent pattern is formed by selectively damaging thecrystal on the surface in the form of the pattern to provide crystaldefects that are preferentially etched to form porous silicon.

The crystal can be damaged by scratching the surface with a sharpinstrument such as a diamond scribe. A pattern could be formed bystamping or translation of a scribe in the x-y axis controlled by aservomechanism. Higher resolution damage patterns can be formed byapplying energetic beams to the surface.

The energy of the beams determine the depth of damage in the surface ofthe crystal. Implantation of ion such as at energies above 10 keV suchas silicon and boron produces deep patterns of damage greater than 5000Angstroms deep. Since most devices can be formed with films of poroussilicon from 100 Angstroms to 5000 Angstroms deep, less energetic beamssuch as plasma and ion milling with energies with energies from about100 volts to 1000 volts are preferred. Suitable ion milling beams cancomprise argon and plasma beams can contain CF₄ or Ar ions. The ion doseis typically above 10¹⁴ cm⁻² suitably from 1 to 5 10¹⁵ ions cm⁻² inorder to form crystal defects that can be converted to porous silicon atan acceptable rate.

The ion beam tools can be translated across the surface of the chip todirectly write a damage pattern. For purpose of automation and costefficiency, it is preferable to form the damage patterns by openingwindows in a photoresist mask layer opaque to an ion beam bylithographic techniques. The photoresist can be a positive or negativephotoresist. The uncured areas of the mask are removed with solvent toform windows exposing the silicon substrate. An energetic ion beam isthen applied to the surface of the silicon substrate exposed through thewindows. The mask is then removed and the surface exposed to etchant.The damage pattern quickly reacts to form porous silicon.

The etchant conditions for preferentially forming porous silicon in theportions of the layer containing the crystal with defects are notcritical. Etching for SOI structures is conducted until thecrystallization of the porous silicon film is at least 50%. Amorphousstructure above 60% to about 80% is necessary for visible roomtemperature luminescence. The etchant contains aqueous hydrofluoric acidin which the HF concentration is at least 5% by weight, usually 20-80%by weight. Chemical stain etchants which do not require anodization ofthe silicon, usually contain a minor amount of an oxidizing acid such asnitric, nitrous or chromate, usually 0.1 to 40 g of oxidizing acid per100 g of HF. The oxidizing acid can be added as the acid or as a watersoluble salt such as a sodium salt of the NO₃ ⁻¹ NO₂ ⁻² or CrO₃ ⁻² ions.Preferred stain etchants are based on the HF:HNO₃ :H₂ O system in whichthe ratios of HF: HNO₃ :H₂ O are preferably from 0.5-8:1-5:10.

The silicon substrate is preferably a highly pure monocrystallinematerial absent any significant number of crystal defects. The substratemay be a chip cut or sliced from a rod or ribbon pulled from a melt. Thesubstrate can also be a layer of crystalline silicon deposited on a baseof another material. The silicon can be n-type or p-type and can bedoped with atoms such as boron or phosphorous. The resistivity of thesilicon crystal is typically from 0.01 ohm-cm to 5 ohm-cm.

Referring now to FIG. 1a, a silicon chip 10 containing a photoresistlayer 12 and a mask 14 is placed in a holder 16 under a light source 20.The mask 14 may be in contact with the resist layer 12 or may be spaceda distance from the resist layer 12.

When the power supply 22 is turned on, the beam 18 generated by thelight source 20 penetrates the window 19 in the mask 14 and reacts withthe resist layer 12 to cure the resist 21 underlying the opening 19 inthe mask. The mask 14 is removed and as shown in FIG. 1b, the chip 10and resist 12 layer are exposed to solvent 28 in tank 30. The solventdissolves the negative resist exposed to the light to form a windowpattern 32 in the resist layer exposing the surface 34 of the chip 10.

The chip 10 is then placed in a holder 42 in an ion beam chamber 40. Anion beam source 44 is activated to radiate a beam 47 at the resist layer12. The beam enters the window pattern 32 to form a latent damagepattern 46 in a layer adjacent the surface. The resist layer is thenremoved by solvent or etching in gas and the chip is placed in a bath 48containing stain etchant 50 as shown in FIG. 1d. A pattern 52 of poroussilicon is formed as shown in FIG. 1e.

The invention will now be illustrated by examples illustrating theselective formation of patterns of porous silicon films on the surfaceof single crystal silicon substrates.

Most of the films were produced by immersion of Si substrates insolutions of HF:HNO₃ :H₂ O with ratios of either 1:5:10 or 4:1:5 byvolume. Reagents used were standard electronic-grade 49% HF and 70-71%HNO₃, and the water was deionized. Some films were also produced insolutions of either 2 grams of NaNO₂ in 100 ml of HF or 0.2 grams ofCrO₃ in 100 ml of HF. Etching was carried out in polypropylene beakerswith no intentional heating of the solutions, and etch durationstypically ranged from 30 seconds to 10 minutes. The etching of Si inHF:HNO₃ :H₂ O solutions is autocatalyzed by HNO₂, so that in freshsolutions a quiescent period of several minutes can persist beforesignificant etching occurs. In order to accelerate the process, thesolutions were usually primed by briefly etching a piece of Si in aconcentrated solution prior to adding the deionized water. The solutionthen contained sufficient HNO₂ that staining of subsequent samples beganmuch sooner. Silicon wafers with several different combinations ofdopant type, resistivity, and crystallographic orientation were used assubstrates, as delivered by the manufacturers (without additionalcleaning). Under some conditions of etching etchant gas evolved andpoorly controlled brown-black films were formed. Stain films wereobserved visually to luminesce red to orange at room temperature underultraviolet irradiation at 365 nm. In addition, all stained films etchedin these solutions were observed to be hydrophobic.

Wafer types etched in HF:HNO₃ :H₂ O solutions include 0.05 Ω-cm B-dopedSi(100), 0.05 Ω-cm B-doped Si(111), 1-10 Ω-cm B-doped Si(111), 1-3 Ω-cmB-doped Si(100), 3-5 Ω-cm P-doped Si(111), and 1-3 Ω-cm P-doped Si(100).

Stain films were formed in HF:HNO₃ :H₂ O solutions without visible gasevolution at the Si surface. In this case, the films exhibit color bandswith increasing thickness, as observed for SiO₂ films on Si, forexample. In particular, under white light the samples sequentiallyappear brown, blue, clear (silicon colored), yellow, etc., withincreasing etch duration. To the naked eye, under ultravioletirradiation the samples are first observed to luminesce a dull red whenthey appear clear in white light, then to luminesce orange when theyappear light yellow in white light, and finally to luminesce brightorange when they are thicker. This film exhibits a much more planarsurface than the brownish-black films as well as uniform thickness,though occasional dips in the interface were observed. Photoluminescencespectra of samples which display various colors under white light areshown in FIG. 2.

FIG. 2a is a photoluminescence spectrum of a film produced by etching0.05 Ω-cm B-doped Si(100) in 1:5:10 HF:HNO₃ :H₂ O, FIGS. 2b-d arespectra of films produced by etching 1-3 Ω-cm B-doped Si(100) in 4:1:5HF:HNO₃ :H₂ O, with spectra taken in brown/dark blue, blue/light blue,and yellow regions, respectively, and FIG. 2e is the spectrum of a filmproduced by etching 0.05 Ω-cm B-doped Si(111) in CrO₃ in HF, this filmdid not luminesce. The sharp peaks at approximately 5630 and 8140Åoriginate from the laser radiation scattered off the surface of thesample.

X-ray photoelectron spectroscopy (XPS) analysis of the brownish/blackand yellow samples show that the porous film is essentially silicon andis amorphous. Pores appear to be in the nanometer range.

EXAMPLE 1

Local damage was created on the surface of a B-doped silicon wafer. Thewafer was immersed in a HF:HNO₃ :H₂ O (4:1:5) by volume etchant. Thedamaged area formed a porous silicon stain film very quickly, before avisible film formed on the wafer as a whole. The stained region wasverified to luminesce in the red under ultraviolet radiation.

EXAMPLE 2

B-doped silicon wafers were coated with photoresist and patternedlithographically to open windows in the photoresist layer.

EXAMPLE 3

Silicon wafers produced in Example 2 were subjected to a plasma etchingin CF₄ and Ar to form a damage pattern in the silicon crystal under thewindow. The plasma-etched regions were then etched as in Example 1.Porous silicon formed faster in the plasma-damaged regions butselectivity was not great enough to allow formation of light-emittingsilicon in the damaged regions before etching occurred in the undamagedregions of the surface.

EXAMPLE 4

Silicon and boron ions were implanted through the window of the resistlayer of the chips of Example 2 into the silicon surface at an energy of15 keV and at a dose of 10¹⁵ cm⁻². The samples were then etched underthe conditions of Example 1. The sample implanted with boron wasilluminated under both white and ultraviolet light. Under ultravioletlight the porous silicon pattern luminesced red-orange to the naked eye.

EXAMPLE 5

The surface of the silicon ship exposed through the window to an argonion milling beam at about 100 V to form a damage pattern having a depthbelow 1000 Angstroms. The surface of the chip was then etched under theconditions of Example 1. The damaged pattern selectively etched to forma light-emitting, porous silicon pattern.

The selective formation of light-emitting porous silicon wassuccessfully conducted under a variety of implant conditions. Theprocess of the invention is capable of forming finely detailedlight-emitting porous silicon patterns in the micron range, the smallfeatures of which are only limited by photolithography.

It is to be realized that only preferred embodiments of the inventionhave been described and that numerous substitutions, modifications andalterations are permissible without departing from the spirit and scopeof the invention as defined in the following claims.

We claim:
 1. A method of forming a pattern of porous silicon which emitsvisible light in a surface layer of single crystal silicon, comprisingthe steps of:depositing a photoresist layer on the single crystalsilicon surface; forming a window in the shape of said pattern in saidphotoresist layer; applying an energetic beam capable of causing defectsin said single crystal to the surface of the photoresist layer, wherebythe beam enters the windows and damages a layer of the crystal on saidsurface in the shape of said pattern; removing said photoresist layer;and selectively etching said damaged crystals in said pattern while notetching undamaged single crystal silicon adjacent said pattern byapplying an etchant to said surface whereby said pattern of crystaldefects selectively etches to form a pattern of light emitting poroussilicon having at least 60% amorphous structure.
 2. A method accordingto claim 1 in which the structure of said porous silicon is at least 70%amorphous.
 3. A method according to claim 2 in which the porous siliconemits visible light at room temperature.
 4. A method according to claim1 in which said surface is etched in a stain etchant.
 5. A methodaccording to claim 4 in which the etchant comprises aqueous hydrofluoricacid containing a minor amount of oxidizing material.
 6. A methodaccording to claim 5 in which the oxidizing material is selected fromacids or salts of CrO₃, NO₂ or NO₃ ions.
 7. A method according to claim6 in which the oxidizing material is HNO₃.
 8. A method of forming apattern of porous silicon which emits visible light in a surface layerof single crystal silicon, comprising the steps of:damaging a layer ofthe crystal on said surface in said pattern by selectively applying anenergetic ion beam containing 1 to 5×10¹⁵ ions cm⁻² capable of causingcrystal defects in said layer to said surface in the shape of saidpattern; and selectively etching said damaged crystals in said patternwhile not etching undamaged single crystal silicon adjacent said patternto form a pattern of light emitting porous silicon having at least 60%amorphous structure.
 9. A method according to claim 8 in which the ionbeam has an energy below 1000 volts.
 10. A method according to claim 1in which the beam is an ion beam.
 11. A method according to claim 10 inwhich the pattern of light emitting porous silicon has a thickness from100 Angstroms to 5000 Angstroms.
 12. A method according to claim 11 inwhich the ion beam contains at least 10¹⁴ cm⁻² ions.
 13. A methodaccording to claim 12 in which the beam is selected from an ion beamcomprising argon and a plasma beam containing CF₄ or Ar ions.